Method for making a plated steel armouring wire for a flexible tubular pipe transporting hydrocarbons, and armoured pipe

ABSTRACT

A process for manufacturing plated-steel armor wires intended for reinforcement of flexible tubular pipes for transporting hydrocarbons, comprising a plating coating is intimately bonded, by high pressure, to a core made of hardenable steel with moderate mechanical properties, and then the plated wire obtained undergoes a rapid high-temperature hardening step followed by a tempering step.

BACKGROUND OF THE INVENTION

The present invention relates to the field of flexible tubular pipes fortransporting hydrocarbons, especially unbonded flexible pipes. Thesepipes are defined in the recommendations API 17J and 17B of the AmericanPetroleum Institute and comprise metal layers and separate polymericlayers, that is to say layers that are not bonded together so as toallow certain relative displacement between the layers.

More precisely, an unbonded pipe of the type intended in the inventiongenerally comprises, from the inside to the outside:

an internal sealing sheath made of a plastic, generally a polymer,resistant to the chemical action of the fluid to be transported;

optionally, a pressure vault resistant mainly to the pressure developedby the fluid in the sealing sheath and consisting of the winding of oneor more interlocked metal profiled wires (which may or may not beself-interlockable) that are wound in a helix with a short pitch (i.e.with a winding angle close to 90° with respect to the axis of the pipe);the profiled wires have a cross section in the form of a Z or a T, orderivatives (theta or zeta) thereof, or a U or an I;

at least one ply (and generally at least two crossed plies) of tensilearmor wires wound with a long pitch—the lay angle measured along thelongitudinal axis of the pipe is, for example, approximately equal to55°; and

optionally, an external protective sealing sheath made of a polymer.

Such a pipe may be what is called a “smooth bore” pipe when the bore isformed directly by the sealing sheath or what is called a “rough bore”pipe when a carcass consisting of an interlocked metal strip wound in ashort pitch is also provided inside the internal sealing sheath, saidcarcass serving to prevent the pipe from collapsing under the externalpressure. When a carcass is used, it is possible for certainapplications to dispense with the pressure vault.

Optionally, the pipe may include, in addition to these layers, otherspecial layers, a metal hoop (wound in a short pitch) and forming partof the pressure vault, intermediate polymeric sheaths, etc.

The precise construction, number and arrangement of the various layersare carefully chosen depending on the applications and the operatingconditions of the pipe, but in all the pipes there are layers formed bywindings of steel reinforcing or armor wires.

For deep-sea applications, which are the intended main applications ofthe invention, the pipe generally comprises all of the following: acarcass, a sealing sheath, a pressure vault, tensile armor plies and anexternal sealing sheath.

Within the meaning of the present invention, the armor wires in questionare the tensile armor wires of the crossed armor plies or else possiblythe profiled wires or the hoop wires of the pressure vault, which willbe called pressure armor wires. By extension, armor wire will also beunderstood to mean a profiled wire which is obtained by the process ofthe invention and would be intended to be used for manufacturing acarcass.

When the pipes are intended to operate in an acid corrosive medium(especially because of the H₂S contained in the effluents transported),which is often called in the oil industry jargon a “sour” medium, it isnecessary to adopt special measures in order to guarantee the corrosionresistance of the armor (tensile and pressure) wires. These measures andthe grades of steels that are necessary are defined in the NACE(National Association of Corrosion Engineers) standard MR01-75 governingthe corrosion resistance in sour medium of steels and alloys.

Usually, steels with good H₂S corrosion resistance have relatively poormechanical properties (R_(m)<850 MPa). Now, if the envisioned operatingconditions are both corrosive and deep sea, it is necessary to preservethe mechanical properties of the metal wires, especially the armorwires, which will be subjected both to corrosion and to the high tensileforces encountered (for a seabed flowline transport pipe, these hightensile forces not perhaps occurring during the life of the pipe oncelaid, but at least while the pipe is being laid). If the steel does nothave very good mechanical properties, it is necessary to increase thesteel thicknesses used, which increases the weight of the pipes, thesize of the winding and laying equipment and therefore the manufacturingcost of the pipes.

According to document FR 2 775 050, which relates to an unbondedflexible pipe intended for static use in a corrosive environment, asteel resistant to H₂S corrosion but with moderate mechanical propertiesis used for the armor wires of the pressure vault, while a steel havinghigh mechanical properties but not resistant to sour corrosion is usedfor the tensile armor plies. This compromise appears acceptable if theH₂S corrosion cannot reach the tensile armor plies; for this purpose, anintermediate H₂S confinement sheath separates the pressure vault, whichwill undergo H₂S corrosion, from the tensile armor plies, which inprinciple will not undergo this corrosion. However, safety is notguaranteed because of the risks of the intermediate sheath beingpierced. Moreover, the poor mechanical properties of the steel used forthe pressure vault mean that they have to be oversized.

In the field of bonded pipes, the document FR 2 569 461 discloses arubber hose intended for transporting corrosive effluents andincorporating, for this purpose, reinforcements consisting of layers ofembedded metal cables, the cables consisting of steel wires coated withplated aluminum (that is to say with intimate bonding to the steel,obtained under high application pressure, for example by coextrusion).This hose, manufactured using the technology of bonded pipes, thereforehas a construction different from that of the unbonded pipes mainlyenvisioned according to the invention, which are subjected to tensilestresses that cannot be envisioned with this bonded pipe technology. Ifthe aim is to retain the beneficial concept of using plated armors inpipes of the type more particularly considered in the present invention,it is necessary to envision plating the metal wire, which has highmechanical properties (R_(m) greater than 1000 MPa and preferablygreater than 1400 MPa), with a corrosion-resistant coating. However, theuse of such high-mechanical-performance metal wires coated with ananticorrosion plating is not entirely satisfactory, especially becauseof a difficulty arising from the fact that the intimate bond between thecoating and the steel is brittle and cannot withstand the stressesassociated with the actual pipe manufacture.

SUMMARY OF THE INVENTION

The object of the invention is to be able to reinforce flexible pipes,especially unbonded pipes, with armor wires that are corrosion resistantbut also have good mechanical properties in order to allow the pipes tobe used at great depth. More precisely, the objective of the inventionis to find a process for intimately coating or plating the steel of thearmor wires that is compatible with the requirements of the use of apipe of the aforementioned type in a sour medium and at great depth.

The invention achieves its objective thanks to a process formanufacturing plated-steel armor wires intended for the reinforcement offlexible tubular, in particular unbonded, pipes for transportinghydrocarbons, of the type in which a plating coating is intimatelybonded, by high pressure, to a steel core, characterized in that thesteel of the core is chosen with moderate mechanical properties and ishardenable, in that the coating is applied to the core and intimatelybonded thereto and then in that in the plated wire obtained undergoes arapid high-temperature hardening step followed by a tempering step, soas to increase the mechanical properties of the plated wire.

The properties of the steel and of the plating coating, and also thetime and temperature of the hardening step are chosen in a linked mannerso as to raise the mechanical properties of the hardened wire withoutdestroying the strength of the plating bond. One of the key factors ofthe invention is the rapidity of the high-temperature heat treatmentwhich, in conjunction with the other parameters, makes it possible tominimize the stresses at the bond between the steel and the coating,especially by preventing the migration of carbon and iron from the steelinto the bond and the coating.

According to the invention, a steel of medium strength is used for thecore of the armor wire, that is to say a steel whose strength R_(m) isbetween 500 and 1000 MPa, advantageously between 800 and 900 MPa. Thismust be a carbon, alloy or low-alloy steel that is “hardenable” (i.e.able to undergo hardening to improve its mechanical properties, thehardening consisting, as is known, of heat treatment hardening:austenization+cooling).

The anticorrosion coating is, for example, made of titanium or titaniumalloys, stainless steel, nickel or nickel alloys.

The plating is carried out cold, by mechanical means, after suitablepreparation (mechanical or chemical deoxidation of the surfaces), usinga technique that allows intimate pressure bonding (for examplecoextrusion or corolling).

After this operation, what is obtained is an assembly characterized by abase/plating bond that is still brittle and by mechanical propertiesdegraded by a plastic deformation (low elongation). At this stage, sucha product would not be compatible with a forming operation in order toserve as armor for a pipe of the type considered in the invention.

According to the invention, the steel thus coated is subjected to a heattreatment comprising a short high-temperature hardening step and atempering step that are carried out so as to minimize the stresses atthe bond between the steel and the coating.

The heat treatment according to the invention makes it possible toimprove the properties of the bond, to restore the ductility propertiesof the plating and to obtain high mechanical properties of the basemetal that are needed in the envisioned applications of this type ofproduct.

This heat treatment is characterized by a short (a few seconds to a fewtens of seconds) high-temperature (900° to 1100° C.) thermal cyclefollowed by rapid cooling and a tempering treatment at a temperature ofaround 400° C. to 700° C., this temperature being adjusted according tothe desired mechanical properties, the tempering step being carried outover a period of a few minutes, advantageously between 10 and 20minutes.

This heat treatment has the following effects, which differ depending onthe constituents of the armor:

in respect of the plating, the treatment restores the ductility withoutcausing any precipitation prejudicial to corrosion resistance;

in respect of the bond, it improves the strength, by relaxation of therolling or extrusion stresses and by metal diffusion. The temperingadditionally improves the ductility, by relaxation of the differentialstresses associated with the tempering step; and

in respect of the base metal, the heat treatment makes it possible toobtain a hardened-tempered structure that combines very high mechanicalproperties (R_(m) greater than 1000 MPa and preferably greater than 1400MPa at least) with a ductility (about 5% elongation) sufficient for theintended applications.

The invention also relates to a flexible tubular pipe for transportinghydrocarbons, which incorporates at least certain armor wiresmanufactured by the aforementioned manufacturing process. Moreprecisely, the invention relates to a flexible tubular pipe fortransporting hydrocarbons, of the type comprising unbonded metal layersand polymeric layers, the metal layers comprising wound armor wires,characterized in that certain of the armor wires are manufacturedaccording to the process of the invention. The invention relatesespecially to a pipe of the type comprising at least a carcass, aninternal sheath, a pressure vault having pressure armor wires, pliesconsisting of tensile armor wires, and an outer sheath, characterized inthat at least certain of the armor wires are manufactured according tothe process of the invention.

Other advantages and features will become apparent on reading thefollowing description, with reference to the appended schematicdrawings,

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a rough-bore pipe to which the inventionapplies;

FIG. 2 is a schematic view illustrating the process for manufacturingthe plated armor according to the invention; and

FIGS. 3A-D illustrate schematically various possible sections of platedarmors.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows a rough-bore pipe which comprises, from the inside to theoutside: a metal carcass 1, generally formed by an interlocked steelstrip wound in a short pitch and intended to withstand being crushedunder the external pressure; a polymeric internal sealing sheath 2, ametal pressure vault, conventionally consisting here of the winding ofan interlocked profiled wire 3 wound in a helix with a short pitch (witha winding angle generally close to 90° with respect to the axis of thepipe), this winding being covered by the winding of a hoop wire 4 woundin a helix of short pitch; armoring 5 resistant to the axial tension inthe longitudinal direction of the pipe and conventionally consisting ofa pair of crossed plies of tensile armor wires 6, 7 wound with a longpitch (typically at an angle of less than 55° to the axis of the pipe);and a polymeric external sealing sheath 8. Other layers, such as anotherarmoring 9 and an intermediate sheath 10, may be provided depending onthe type and application of the pipe.

The invention relates to the plating of the tensile armor wires 6, 7 andalso of the pressure armor wires 3, 4 (as the case may be), according toa treatment that is illustrated in FIG. 2.

The starting wire 20 has a core 21, made of a base metal of moderatemechanical properties (for example R_(m) of 800 to 900 MPa), and acoating 22 made of plating metal. The base metal may, for example, be abase steel of the silicon-chromium (55SiCrV) type in the spheroidizedstate so as to allow plating. The plating metal may for example be anickel base (NiCrMo, series 6x according to the AISI) or a nickel alloy(series 8x). The starting wire 20 passes through a coextrusion die 23,from which it emerges with the coating 22 intimately bonded to the core21. The wire thus plated passes through a station 24 forhigh-temperature heat treatment, typically at least 800° C. andpreferably at least 1100° C., for example by induction heating of thewire. This treatment is rapid (lasting a few seconds to a few minutes atthe very most). Austenization of the base steel takes place during thetreatment, which is followed by a rapid quench in a quenching station 25(for example an air, water or oil quench) which is accompanied by acontrolled martensitic transformation and therefore allows highmechanical properties to be obtained, with an R_(m) of around 2000 MPa.However, the speed of the treatment prevents harmful elements (iron andcarbon) from diffusing into the bond, which would degrade the qualitythereof and would degrade the corrosion resistance of the plating.Induction heating is advantageous not only for its rapidity but alsobecause it gets around the problems associated with any reflection ofthe coating.

The wire thus obtained then passes through a thermal tempering station26, at a temperature of around 450° C. for about 15 minutes.

It is possible, after the corolling or coextrusion plating in thestation 23 and before the heat treatment in the station 24, to providean intermediate tempering treatment, for example allowing the ductilityand the quality of the bond to be restored.

It is advantageous to ensure that the mechanical properties of the core21 and the coating 22 are appropriate, by choosing properties that arerelatively similar. The constituent materials will preferably be chosenso that the difference between their respective mechanical strength(R_(m)) properties is no greater than 200 MPa. By keeping thisdifference between the wire and the coating relatively small, theuniformity of coating thickness distribution is improved and the qualityof the bond obtained is also improved.

It is possible to provide several plating layers. The thickness of theplating is generally around 200 μm to 500 μm. It must be thick enough towithstand mechanical attack and to be corrosion resistant. Itrepresents, in cross section, less than about 10% of the total crosssection of the wire.

FIG. 3 illustrates, by way of examples in FIGS. 3A, B, C and Drespectively, four types of possible cross section of the plated armorwires. The round shape 30 of FIG. 3A is the simplest, but it is alsopossible to envision a rectangular shape 31 of FIG. 3B or a “zeta” shape32 of FIG. 3C or a “theta” shape 33 of FIG. 3D, these shapes forconstituting the pipe reinforcement wires being conventional per se.

1. A process for manufacturing plated-steel wire in which a platingcoating is intimately bonded by high pressure to a steel core, themethod comprising selecting steel for the core having relativelymoderate mechanical properties with a strength R_(m) of between 800 and900 MPa and being a hardenable steel, applying a plating metal coatingto the core, wherein the plating metal is chosen from the groupconsisting of titanium and titanium alloys, stainless steel, nickel andnickel alloys, and selecting the materials of the core and of thecoating so that the difference between their respective mechanicalstrength R_(m) properties is not greater than 200 MPa; and intimatelybonding the coating to the core forming a plated wire and applying arapid high-temperature hardening step to the plated wire, which isaccompanied by a controlled martensitic transformation, and thenapplying a tempering step to the wire.
 2. The process as claimed inclaim 1, wherein the properties of the steel and of the plating coating,and also the time duration and temperature of the hardening step arechosen with reference to the properties of the steel and the coating toraise the mechanical properties of the hardened wire without destroyingthe strength of the plating bond between the core and the coating. 3.The process as claimed in claim 2, wherein the high-temperaturehardening step is carried out at a temperature of around 800° C. to1100° C.
 4. The process as claimed in claim 3, wherein thehigh-temperature hardening step is carried out for a time period of afew seconds to a few minutes.
 5. The process as claimed in claim 1,wherein the high-temperature hardening step is carried out by induction.6. The process as claimed in claim 4, wherein the tempering is carriedout for a time of between 10 min and 20 min.
 7. The process as claimedin claim 1, further comprising performing an intermediate tempering stepbefore the rapid hardening step.
 8. The process as claimed in claim 1,wherein the high-temperature hardening step is carried out at atemperature of around 800° C. to 1100° C.
 9. The process as claimed inclaim 1, wherein the high-temperature hardening step is carried out fora time period of a few seconds to a few minutes.
 10. The process asclaimed in claim 9, wherein the tempering is carried out for a time ofbetween 10 min and 20 min.